1. Field of the Invention
This invention relates to a process and apparatus for reducing macrosegregation in the casting of a plurality of closely-spaced ferrous or non-ferrous metal alloy ingots employing at least one substantially static magnetic field that forms the basis for improved ingots having a fine, equiaxed grain structure and a reduced porosity uniformly throughout each ingot.
2. Brief Description of the Prior Art
It is well known by those skilled in the art of continuously or semicontinuously casting ingots that large ingots of metal alloys, for example some hard aluminum alloys, such as 2124, 7050 and 7075, usually exhibit severe ingot centerline macrosegregation. This means that the central region of the ingot is depleted of major alloying ingredients. This is believed to be due to liquid metal convection in the ingot head which is driven by the incoming liquid metal during the pouring stage, and by the density gradients due to thermal and solutal differences produced during the cooling stage. Such convection may carry dendrite fragments and loose dendritic crystals from the outer portions of the ingot to the center of the ingot. Since those fragments or crystals are depleted in alloying ingredients such as copper, magnesium, and zinc due to their partition ratio, the average composition near the ingot centerline becomes much less than the bulk average. As a result of this macrosegregation, the molten material in the central region of the ingot does not develop the desired mechanical properties which may be necessary for certain heavy section applications. That is, since the alloying ingredients increase strength, this depletion of the alloys results in weakened metal in the center of the ingot.
Economically, it is desirable to cast a plurality of metal alloy ingots simultaneous in one installation. In a typical installation, in which a plurality of ingots are cast simultaneously and continuously withdrawn from the molds, the molds are very close together. Controlling macrosegregation in the simultaneous casting of a plurality of metal alloy ingots to maintain a desired uniform concentration of alloy elements throughout each ingot is of particular importance in the production of high quality metal alloy ingots. Ideally, a separate electromagnetic coil should be placed around each mold or around each ingot. However, in view of the close location of the several molds relative to each other, there is not enough room to provide these separate coils, that is, one for each mold or ingot.
Macrosegregation is a term which is used to describe segregation on a scale which is comparable to the dimensions of the ingot. It is distinct from microsegregation which is on the scale of the spacing between the dendrite branches.
Various processes and apparatus for reducing macrosegregation in the casting of a plurality of metal alloy ingots have been known, and various processes and apparatus have been used for controlling the grain structure of such ingots. However, none teach or suggest the improved results of the process and apparatus of the present invention.
U.S. Pat. No. 1,978,222 discloses a method of producing a metallic material employing a mold. This patent states that a magnetic field is applied separately to a casting mold having molten metal therein. Each mold is placed on a base plate which has apertures through which metallic plugs protrude. The magnetic field is applied separately to each ingot by means of the magnetic plug. This patent does not teach that a coil may be passed around a plurality of casting molds.
U.S. Pat. No. 3,810,504 discloses a method and apparatus for the production of directionally solidified castings employing an annular mold having one or more molding cavities. This patent states that a pair of heat radiating elements are disposed concentrically with the mold, one element being located inside the mold and the other element being located outside the mold. To provide directional solidification, the molds are heated on their sides and a chill is provided at their bottoms. This patent states that the mold assembly includes an outer heat source consisting of a cylinder composed of graphite which is heated by an induction coil that is disposed around the molds.
In spite of these prior art disclosures, there remains a very real and substantial need for a process and apparatus for reducing undesired macrosegregation occurring in the center of an ingot, and particularly in the simultaneous casting of a plurality of metal alloy ingots. Such a process and apparatus is disclosed herein and may be employed to create improved ingots having a refined equiaxed grain structure and a reduced pore size.